A method and system for depleting plant nutrient compounds in open bodies of water by cultivating aquatic plants in a container placed in the body of water. The invention is directed primarily to maintaining high quality fresh water; however, the method and device described herein could be fashioned to effectively maintain other waters such as marsh water and sea water in its natural state.
The depletion of plant nutrients in a controlled fashion permits water quality to be maintained. Desired quality water standards are defined for lake water as potable water able to support sport fish and other organisms with relatively low amounts of aquatic plant growth. Maintenance of these parameters is dependent upon the control of bio-available nutrients that would otherwise support undesired plant growth.
The quality of lake waters is dependent on the availability and utilization of nutrient compounds such as nitrates and phosphates. If aquatic vegetation is allowed to become established, the absorption of nutrients by plant life strips these materials from the water. A satisfactory level of water quality is maintained so long as the population of aquatic plants is healthy and growth is controlled by a limited supply of nutrients. Where the nutrient levels are excessively high due to natural causes, man-made causes or both, the vegetation progressively continues to grow until the surface waters are covered, and thus the uses of the lake are reduced. Swimming and boating activities become limited.
Progressive vegetation leads to two repercussions. One, the aquatic plants will grow to the point that dissolved oxygen levels, just prior to sun up, will be below 4 ppm and the plants will begin to die. Two, the need to clear them from the surface will sponsor the use of herbicides.
In both cases, the same nutrients that these plants stripped from the water will now begin to reenter the water available for the remaining plants or, and much more likely, for algae.
The lake waters will now possess a new abundance of nutrient compounds. The availability of these nutrients will sponsor the growth of whatever aquatic plant or algae that can consume them most rapidly.
The species that will be capable of utilizing these nutrients most effectively are the phytoplanktons. These very small organisms are generally single cell or colonies of independent cells that can double their biomass in as little as 72 hours. Their populations are dense and of a nature that produces the "pea soup" appearance of a lake. They are capable of blocking light penetration and thus the death of all aquatic plants that existed prior to the algae bloom.
From this point forward, no large aquatic plants will survive and the dominant "pea soup" algaes will retain control of the ecosystem. The lake waters will no longer support anything more than minimal fish life. The potential recreational uses of the lake are reduced further from those available prior to the elimination of the large aquatic plants.